Oil at standard temperature and pressure conditions is a liquid and is reasonably dense and suitable for transportation. The market for oil is global in nature because it can be readily transported in tankers and stored in surface storage containers. Because of natural gases gaseous nature, natural gas is far more difficult to transport and store. Most natural gas is transported through pipelines, which means that sources of supply must be local. Gas is usually stored in underground natural caverns and the storage locations are therefore tied to the availability of these caverns.
A need has exists to link a subsea storage facility near the subsea pipelines in an economical manner, that is easy to monitor and with few moving parts for failure.
The main complication in development of a global gas industry is that at standard temperature and pressure (stp), gas is extremely diffuse and therefore has very little economic value for a given volume compared to oil (a difference of three orders of magnitude at $7/MCF for gas and $50/BBL for oil). Due to this difference in value per volume, combined with the gaseous state, transport of gas over long distances at stp is not economically feasible. Various methods for achieving more favorable ratios of gas value for a given volume are commonly used to make the transmission and storage more economically attractive, such as compressing or liquefying it. Compression is the most commonly used method for transportation because it is the preferred method of use in pipeline systems. Both methods can be used for marine transportation wherein liquefaction is used for Liquified Natural Gas (LNG) and compression is used for Compressed Natural Gas (CNG).
A measure of the economic feasibility for storing and transporting gas is the volume ratio, defined as volume of gas that can be stored in a given volume in its compressed condition divided by the volume of gas that could be stored in the same volume at standard temperature and pressure. LNG has a volume ratio of roughly 600, which means that its economic value per cubic meter is roughly a factor of two less than that of oil at the price conditions listed above. Although CNG has not been used to date, it has some applications where it may be a better method than LNG, depending on the gas conditions and the distance from the end user. CNG can achieve ratios of roughly 300, or roughly a factor of two less than that of LNG but has advantages because the facilities required at both source and destination are simpler than those required for LNG.
Both LNG and CNG require some means to get the gas back to standard pipeline conditions once the gas has reached its desired destination. Both LNG and CNG have severe complications storing gas after delivery. For LNG this is addressed by building either pressurized or cryogenic storage containment tanks onshore. Both methods are expensive and dangerous. CNG has not been used to date, but one of the main reasons could be the lack of availability of efficient storage means.
Various oil storage systems have been deployed on the seafloor, namely the Harding platform in the North Sea and the Dubai Oil Storage tanks in the Middle East. Additionally, oil over water storage systems have been deployed from Gravity Based Structures in the North Sea. Whereas oil has been stored on the seafloor for many years and mainly as a matter of convenience, storage of gas on the seafloor has never been done and yet has some very important technical advantages over gas storage through other methods. Additionally, gas storage on the seafloor can be an enabling technology for some of the CNG applications that are currently contemplated.
All presently available means for storing natural gas are dangerous with significant potential for both environmental damage and loss of life and property. Underground natural salt caverns are typically used for low pressure storage of natural gas. There have been many accidents related to these caverns, including both fires and explosions. LNG storage tanks have also had major accidents resulting in disastrous consequences. As both LNG demand and population along the shores increase, it has become increasingly difficult too locate LNG regassification units for permitting reasons despite the large market need.
The proposed gas storage invention can serve in several applications.
A need exists for a system for storing natural gas which is produced during a well testing operation offshore wherein the oil and gas operator does not want to commit to building a pipeline for gas export before the reservoir has been producing for long enough to evaluate its characteristics and condition.
A need exists for a system that can store significant volumes of gas and yet can be readily deployed and reused at the end of its initial service. This system has to be both portable for second and additional use applications and the design must also be capable of addressing all anticipated applications including a range of water depths and pressures.
A need exists for a gas storage system that can be built in a desired location close to a pipeline network and independent of the prior existence of naturally occurring caverns.
A need exists for a gas storage system which is remote from human life and property.
A need exists for a gas storage system that is not prone to catastrophic gas release and consequent fire or explosion.
A need exists for a gas storage system that can be used in conjunction with any of the proposed CNG or LNG systems in order to decouple the depressurization conditions from the immediate pipeline needs.
The present embodiments meet these needs.
The present embodiments are detailed below with reference to the listed Figures.